Question

How would you explain an atomic mass number?

Answer 1

Well it's the sum of the massive nuclear particles, protons and neutrons.

Explanation:

As you know, a nucleus contains a given number of nuclides: (i) protons, massive positively charged particles; and (ii) neutrons, massive neutrally charged particles. The associated electronic mass is fairly insignificant in comparison.

The number of protons gives `Z`, the atomic number, which number defines the element: `H, Z=1`; `He, Z=2`; `Li, Z=3`; `Be, Z=4`..................

However the nucleus may contain different numbers of neutrons, which difference gives rise to isotopes. For most hydrogen nuclei, where `Z=1` by definition, there is no neutron, and we represent this isotope as `""^1H`, the protium isotope. A small number of hydrogen nuclei contain 1 neutron, and we represent this isotope as `""^2H`, the deuterium isotope. A smaller number of hydrogen nuclei contain 2 neutrons, and we represent this isotope as `""^3H`, the tritium isotope.

The weighted average of all isotopes is the mass number, and this number is quoted on the Periodic Table; of course you will never be expected to remember these numbers in an exam. You will be expected to utilize the Periodic Table provided to explain atomic numbers and mass numbers.